The present invention relates generally to the field of optical communications and more particularly to the field of coupling of light into and out of optical waveguides.
The advantages of using optical transmission systems for communications are well recognized. Optical waveguides comprising dielectric fibers having a transparent core coaxially surrounded by a cladding material of lower dielectric index may be used to guide and transmit optical signals over long distances in optical communications systems. Since signals are transmitted at optical frequencies, these optical fibers are characterized by extremely large bandwidths and hence are capable of handling large amounts of information. Furthermore, by proper design, optical fibers are capable of supporting multi-mode propagation. Each propagating mode is independent of other propagating modes and can be used for the transmission of independent signals. Due to their relatively small physical size and large bandwidth capabilities, optical fiber communication systems offer attractive alternatives to conventional communication systems in many applications.
Typically, in optical communication systems, light from a laser, light emitting diode (LED) or some other source is coupled into the end of an optical fiber. The light propagates through the fiber core and can be detected at the opposite end of the fiber, to form a point-to-point communication system. Specific propagating modes can be launched into the fiber to carry independent signals but this generally requires expensive optics systems and precise alignment of the light source used for each mode with the end of the fiber. Such optics systems and alignments are not always easily adaptable to multi-mode coupling. Furthermore, if it is desired to communicate with intermediate stations along the length of the fiber, it is generally necessary to physically interrupt the fiber, detect the various propagating modes, strip out the desired mode and relaunch the remaining modes back into the fiber to continue their propagation to the other end or to the next intermediate station where this process is repeated. This is a significant disadvantage of optical fiber communication systems when it is desired to communicate with a number of intermediate stations along the length of the fiber.
One method of avoiding this disadvantage to some extent, is to employ bundles of many fibers, with a separate fiber connecting each of the various intermediate stations with every other intermediate station. This eliminates the need for the expensive optics systems required at each station to couple to unwanted modes but has the further disadvantages of requiring the use of many fibers and not fully utilizing the entire capacity of each fiber.
In the area of coupling light into thin-film integrated circuit optical waveguides, some work has been done utilizing holograms to form phase gratings on the flat face of a thin-film waveguide. The hologram serves to convert an incident light beam into a planewave whose direction of propagation is such as to excite a predetermined mode in the waveguide. Typical is U.S. Pat. No. 3,885,856 to Ostrowsky et al. which discloses a holographic method for implementing such a coupler and U.S. Pat. No. 3,864,016 to Dakss et al. which discloses a similar coupler, adaptable to coupling light from the end of a number of optical fibers into integrated optical circuits. These couplers however, are useful only with integrated circuit optical waveguides.
It is desirable to have a coupler for optical fiber waveguides which will overcome the aforesaid disadvantages and it is to this end that the present invention is directed.